The present invention relates to a transmission control device comprising a main transmission and a sub transmission.
A transmission having a main transmission such as an automatic transmission and a manual transmission, a speed-change mechanism and a sub transmission which selectively attain the gear ranges of the speed-change mechanism has conventionally been known. In this case, for example, a planetary gear device is used for the speed-change mechanism of the sub transmission, the input shaft of the sub transmission is connected to a sun gear of the planetary gear device, the output shaft of the sub transmission is connected to the carrier of the planetary gear device, and a clutch for engaging and releasing the sun gear and the carrier and a brake for engaging and releasing a ring gear are provided, the clutch being engaged and the brake being released to effect a direct-coupling range and the clutch being released and the brake being engaged to effect a reduced-speed range. The engaging and releasing of the clutch and the brake are changed over by switching the working oil of the hydraulic servomotor of the frictional engagement devices of the clutch and the brake by means of the manual shift valve which is controlled by automatic operation and a low-high switching valve which is controlled by hydraulic pressure corresponding to the vehicle speed and which sets the gear ranges of the sub transmission. The low-high switching spool of the low-high switching valve which switches the working oil between the hydraulic servomotors of the frictional engagement devices of the clutch and the brake is provided for receiving hydraulic pressure (a governor pressure) corresponding to the vehicle speed at one end, and thus, when the direct-coupling range is shifted to the reduced-speed range in the case of a high vehicle speed, setting of the reduced-speed range and overreving of the engine are prohibited by the fact that the low-high switching spool receives the hydraulic pressure corresponding to the vehicle speed at one end, whereby unsuitable speed reductions are prevented. An inhibitor valve (a plunger) which sets the low-high switching spool to the direct-coupling range regardless of the hydraulic pressure corresponding to the vehicle speed when the manual shift valve is shifted from the reduced-speed range to the direct-coupling range is provided at one end of the low-high switching spool of the low-high switching valve.
In the prior art, for example, when the manual shift valve is changed from the reduced-speed range to the direct-coupling range as described above, the low-high switch spool is set at the direct-coupling range regardless of the hydraulic pressure corresponding to the running state of the vehicle such as vehicle speed, and thus it had the disadvantage that, when a driver makes a mistake in shift operation and shifts from the reduced-speed range to the direct-coupling range to set the direct-coupling range, for example, during reduced-speed running such as in climbing an incline in which the reduced-speed range is set and a large torque occurs in the driving wheels, the direct-coupling range is set regardless of the running state of the vehicle. Since the hydraulic pressure applied to the conventional low-high switching spool is the governor pressure corresponding to the vehicle speed, it is impossible to freely switch the setting of the transmission and the set speed is disadvantageously switched by the change in the governor pressure.